An opposite role for tau in circadian rhythms revealed by mathematical modeling

Abstract

Biological clocks with a period of Ϸ24 h (circadian) exist in most organisms and time a variety of functions, including sleep-wake cycles, hormone release, bioluminescence, and core body temperature fluctuations. Much of our understanding of the clock mechanism comes from the identification of specific mutations that affect circadian behavior. A widely studied mutation in casein kinase I (CKI), the CKI tau mutant, has been shown to cause a loss of kinase function in vitro, but it has been difficult to reconcile this loss of function with the current model of circadian clock function. Here we show that mathematical modeling predicts the opposite, that the kinase mutant CKI tau increases kinase activity, and we verify this prediction experimentally. CKI tau is a highly specific gain-of-function mutation that increases the in vivo phosphorylation and degradation of the circadian regulators PER1 and PER2. These findings experimentally validate a mathematical modeling approach to a complex biological function, clarify the role of CKI in the clock, and demonstrate that a specific mutation can be both a gain and a loss of function depending on the substrate.kinase ͉ systems biology ͉ phosphorylation ͉ PER ͉ degradation C ircadian rhythms govern key physiologic processes including sleep-wake cycles; glucose, lipid, and drug metabolism; heart rate; stress and growth hormones; and immunity, as well as basic cellular processes such as timing of the cell division cycle (1-6). The disruption of circadian rhythm causes significant physiologic stress, is frequently experienced in jet lag and night-shift work, and has been linked to bipolar disorder (7). Thus, circadian regulation of physiology has important consequences for health. A detailed quantitative model that makes clear, testable, and accurate predictions about the clock and how we may manipulate it can therefore have benefits for human health.Much of our understanding of clock components and their interactions began with the identification of mutations that affect circadian behavior (8, 9). In mammals, the original and most extensively studied circadian rhythm mutation is the semidominant tau, first described in 1988. Hamsters with this mutation show phase-advanced activity and have a circadian period of 20 h when homozygous mutant animals are isolated from time cues (9). This tau mutation has been identified as a missense mutation within the substrate recognition site of casein kinase I (denoted CKI tau ) (10). CKI and the closely related CKI␦ are widely expressed serine-threonine protein kinases implicated in development, circadian rhythms, and DNA metabolism (11). When tested in vitro on multiple substrates, CKI tau was shown to have a much reduced overall catalytic activity (10,12,13). This partial loss-of-function mutation and its phenotype have been difficult to reconcile with our current understanding of the molecular feedback loop that governs timing in mammalian cells (13) and recent empirical observations on clock function (14-16). For example, Dey et al. (1...

Role of Phosphorylation in the Mammalian Circadian Clock

“…In line with these results, Gallego et al (2006b) found a higher destabilizing effect of the tau kinase on PER1 and PER2 in cell culture. The starting point of their experiments was mathematical modeling, which claimed an increased kinase activity (gain-of-function) for CKIε(tau) in vivo as the only possible explanation for the shortperiod phenotype.…”

“…The starting point of their experiments was mathematical modeling, which claimed an increased kinase activity (gain-of-function) for CKIε(tau) in vivo as the only possible explanation for the shortperiod phenotype. With respect to the tau mutation, our model and that of Gallego et al (2006b) offer alternative but not mutually exclusive explanations. We favor a mechanism where a reduced kinase activity of mutant CKIε leads to an altered, less nuclear localization of PER proteins (as observed by Dey et al 2005).…”

“…We favor a mechanism where a reduced kinase activity of mutant CKIε leads to an altered, less nuclear localization of PER proteins (as observed by Dey et al 2005). Because phosphorylation-induced proteasomal degradation of PER proteins is primarily a cytosolic event (Vanselow et al 2006), the overall phosphorylation of the PER proteins in tau hamsters may even increase (as observed by Gallego et al 2006b). We propose that this increased phosphorylation is primarily due to a higher amount of PER substrate in the cytoplasm, rather than to an increased kinase activity.…”

NAN-190 potentiates the circadian response to light and speeds re-entrainment to advanced light cycles

“…Since both per mRNA and Per protein levels are correlated with behavioral phase shifting (Best et al, 1999;Masubuchi et al, 2005), it is possible that the greater phase advance to light observed with MKC-242 and NAN-190 may be due to their extended elevation of per, and thus Per protein. Such might be accomplished by allowing enhanced Per production from the lightinduced per mRNA or by affecting the stability of Per protein (which is known to alter phaseshift magnitude) (Gallego et al, 2006;Jakubcakova et al, 2007;Lee et al, 2007;Masubuchi et al, 2005). Naturally, there are many other potential targets in addition to per and Per protein and at this point in time, they cannot be ruled out.…”

Dependence of the period on the rate of protein degradation in minimal models for circadian oscillations

“…Among the multiple molecular mechanisms that influence the period of circadian rhythms, a major role is played by those that control clock protein degradation (Liu et al 2000;Eide et al 2005;Gallego et al 2006;Busino et al 2007;Reischl et al 2007). Here we compared three minimal models proposed for circadian clocks, showed that they predict opposite profiles of period as a function of the clock protein degradation rate, and indicated how to reconcile these contradictory predictions.…”